1. Technical Field
The present invention relates generally to microcomputers and more particularly to a battery-driven microcomputer having a standby mode in which initial resetting is prevented from being inadvertently performed and thereby rendering power consumption reducible.
2. Background Art
One-chip microcomputers are widely used as control circuits in cameras, home electric appliances and other kinds of electronic equipment, as remote controllers and the like. These are generally 4- or 8-bit microcomputers.
One-chip microcomputers of this sort are different from general purpose microprocessors in that one chip incorporates a number of elements such as a register, a memory, ALU and the like that are connected to a common bus. While controlling circuits including an I/O buffer, RAM, a register, ALU and the like, the controller such as a built-in CPU exchanges data with an external circuit via the I/O buffer.
Most of the one-chip microcomputers (one-chip microprocessors) are battery-driven and provided with a standby mode to economize battery power consumption. Each circuit of a one-chip microcomputer is usually set on standby mode and an operation mode is reset in response to keyboard entry and other mechanical operations. Some of the one-chip microcomputers are so controlled as to preserve data in the operating mode immediately preceding the standby mode or at a point of standby time and to restart operations in the state immediately preceding the standby mode or a designated state when the operation mode is followed. Various home electric and electronic appliances set to operate by means of time switches, for instance, have built-in one-chip microcomputers which function as those causing prescribed control to be exerted at a preset time and the time to be displayed simultaneously. Under that control, time data and what is used for exerting desired control are maintained even in the standby mode. Moreover, a built-in clock keeps counting the time so that each point of time may be indicated. Under the control of this kind, the data stored in the program counter, the registers and the memory contained in the microcomputer are kept intact during the standby mode and a specific operation is kept on as occasion demands.
Moreover, the information given by pressing a specific key one time earlier is stored in the program counter and the registers of a remote controller and held even in the standby mode that follows. When the key is pressed subsequently to restore the operation mode, the information stored immediately prior to the mode standby causes control to be exerted again from the state prior to the standby mode.
Although supply voltage in the operation mode is normally about 5 V, the supply voltage itself is reduced to about 3 V in the standby mode and the operation of each circuit is also set to a halt condition. It is thus attempted to economize the power consumption during the mode standby further. If, however, the battery of a microcomputer designed to reduce the supply voltage to a desired standby level, 3 V for instance, is replaced during the standby mode, initial resetting may be performed as the supply voltage lowers to an initial reset voltage because of insufficient power backup. Moreover, there is also fear that the initial resetting will be performed when the supply voltage lowers during the standby mode as the battery power consumption increases and this arises as long as an initial reset circuit operating upon detection of the supply voltage is employed. When the initial resetting is performed, the data held in the register and the program counter are lost.